1. Field of the Invention
The invention relates to a controller for a variable valve device (hereinafter, also referred to as “variable valve device controller”) of an internal combustion engine, the variable valve device including a hydraulic phase changing mechanism that changes valve timing, and a phase locking mechanism that locks the valve timing to a specific phase. More specifically, the invention relates to a variable valve device controller that controls the operation of the phase changing mechanism.
2. Description of Related Art
In a variable valve device controller described in Japanese Patent Application Publication No. 11-210424 (JP 11-210424 A), from the viewpoint of the stability of feedback control of valve timing, the valve timing is locked to a specific phase by a phase locking mechanism when the temperature of hydraulic oil in a phase changing mechanism is low. That is, when the temperature of hydraulic oil in the phase changing mechanism is low, the controller executes a control to limit the operation of the phase changing mechanism.
When the operating speed of the phase changing mechanism is compared between a case where a kind of hydraulic oil with high viscosity is used and a case where a kind of hydraulic oil with low viscosity is used, assuming that the hydraulic oil temperature and the engine speed are the same between the two cases, the operating speed is lower in the former case than in the latter case owing to the greater resistance acting on the hydraulic oil with the higher viscosity. That is, the operating speed of the phase changing mechanism varies under the influence of not only the temperature of hydraulic oil but also the viscosity of hydraulic oil based on the difference in the kind of hydraulic oil or the like.
The variable valve device controller described in JP 11-210424 A limits the operation of the phase changing mechanism indiscriminately when the temperature of hydraulic oil is low. Accordingly, it is not allowed to change the valve timing even when a low viscosity kind of hydraulic oil is used, in other words, even when the influence of the viscosity of hydraulic oil on the operating speed of the phase changing mechanism is predicted to be small.
This is equivalent to a state in which even though it is allowed to change the valve timing in accordance with the engine operation state, the variable range of the valve timing is limited excessively relative to a variable range suited for the engine operation state.
While the present case is directed to locking of the operation of the phase changing mechanism by the phase locking mechanism as an example of a limitation on the operation of the phase changing mechanism, presumably, the same problem mentioned above arises as long as the variable valve device controller used is one that executes a control to limit the operation of the phase changing mechanism by taking the temperature of hydraulic oil into consideration.